1. Field of the Invention
The present invention relates to a method of drilling a through-hole for a double-sided or multilayered printed wiring board. More specifically, it relates to a method of drilling a through-hole, in which a water-soluble lubricant is placed so that the generation of heat in a drill bit is prevented in drilling a hole and a high-quality hole is made with high efficiency.
2. Description of the Related Art
U.S. Pat. No. 4,781,495 and U.S. Pat. No. 4,929,370 disclose methods of drilling a hole for inter-surface conduction in a laminate produced by attaching a metal foil on an insulating material, with a water-soluble lubricant placed on one surface, or front and reverse sides, of the laminate. It is disclosed that these methods use a sheet prepared by impregnating paper, etc., with a mixture of a glycol such as diethylene glycol or dipropylene glycol, which is a solid water-soluble lubricant, with a rigid wax such as fatty acid or a nonionic surfactant.
However, the problem of the above methods is that the effect on the prevention of heat generation in a drill is insufficient, in that the porous sheet is only poorly impregnated with the above mixture of that the sheet is sticky.
JP-A-4-92488 to JP-A-4-92493 disclose methods of drilling a hole in a laminate with a specific water-soluble lubricant placed on the laminate. It is disclosed that in these methods the water-soluble lubricant is selected from polyethylene glycol having a weight average molecular weight of 600 to 9,000, monoether of polyoxyethylene, ester of polyoxyethylene, monoester of polyoxyethylene sorbitan and a polyoxyethylene-propylene block copolymer.
These methods show some improvement, while the effect on the prevention of heat generation in a drill is not satisfactory.
Further, JP-A-4-92494 discloses a water-soluble lubricant sheet formed from 20 to 90% by weight of a polyethylene glycol having a weight average molecular weight of at least 10,000 and 80 to 10% by weight of a water-soluble lubricant.
The above water-soluble lubricant sheet shows a sufficient improvement in the effect on the prevention of heat generation in a drill, while the defect with this sheet is that it is liable to break so that it is difficult to handle it.